1. Field of the Invention
The present invention relates to an electro-optical device represented by an EL (electroluminescence) display device comprising a semiconductor element (an element using a semiconductor thin film, typically, a thin film transistor) that is formed on a substrate, and to an electronic device (electronic equipment) having the electro-optical device as a display. Specifically, the invention relates to a method of manufacturing these devices.
2. Description of the Related Art
In recent years, remarkable progress has been made in a technique for forming a thin film transistor (hereinafter referred to as a TFT) on a substrate, and developing the application of TFTs to active matrix type display devices is proceeding. TFTs using a polysilicon film, in particular, have a higher electric field mobility than that of conventional TFTs using an amorphous silicon film, and hence are capable of high speed operation. Accordingly, control of pixels, which in the past has been made by driver circuits outside the substrate, now can be made by driver circuits formed on the same substrate on which the pixels are formed.
The attention is kept drawn to such active matrix type display devices in expectation of various advantages, such as lowering of production cost, downsizing of display devices, and an increase in yield and throughput, as a result of putting all the various kinds of circuits and elements onto the same single substrate.
In active matrix type EL display devices, every pixel is provided with a switching element formed of a TFT, and an EL layer emits light when a drive element that controls current is operated by this switching element. The mainstream at present of the structure of the EL layer is the layered structure with three or four layers, proposed by Tang, et al. from Eastman Kodak Company.
Lately, however, many are concerned that multi-layering of the element structure will invite complication of the manufacture process and increase of the production cost. Attempts have been made in this connection to reduce the number of layers by doping a light emitting layer with a specific impurity element and using it as an electron conveying layer or an electron injection layer.
For instance, Idemitsu Kosan, Co. Ltd. proposes doping the light emitting layer (distylarylene derivative) near its surface with cesium (Cs) that has a small work function to use the doped region as the electron conveying layer. (Preliminaries for the Sixth FPD Seminar, sponsored by Electronic Journal, pp. 83-88)